1. Field of the Invention
The present invention generally relates to determining target elevation angle, altitude and range and the like in a monopulse radar system and, more particularly, to a method and apparatus for determining target elevation angle, altitude and range and the like in a monopulse radar system with reduced multipath errors relative to conventional apparatuses and methods.
2. Description of the Related Art
FIG. 1 is a diagram for explaining causes of multipath errors in radar systems. A radar system 1 generates an antenna beam having an axis of symmetry about a line defined by an angle .theta..sub.1 relative to some reference plane. In this case, the reference plane is parallel to the surface of the earth. Depending upon the beam-width of the antenna beam of the radar system 1, the antenna beam transmitted to the target 2 may be transmitted directly along the axis of symmetry at angle .theta..sub.1 or may be transmitted to the target 2 by reflection at an angle .theta..sub.2 relative to the surface of the earth. Also, a reflection or radar return signal termed an `echo` may be received at the radar system 1 either directly along the axis of the symmetry of the antenna beam at angle .theta..sub.1, or indirectly by reflection at an angle .theta..sub.2 relative to the surface of the earth. The different transmission and/or reception paths cause multipath errors in target elevation angle, altitude and range measurements and the like. This is because, as target 2 moves relative to radar 1, direct echoes and multipath echoes go into and out of phase repeatedly. The problems caused by this is illustrated in FIG. 2.
FIG. 2 is a polar plot of target elevation angle versus sum and difference gain amplitudes for a monopulse radar which is receiving strong multipath echoes. In the absence of the multipath effects, the beam pattern received by a monopulse radar system would have amplitudes defined by envelopes formed by connecting the peaks of the sum and difference signals, respectively. The sum and difference signals are well-known phenomena occurring in monopulse radar systems.
Due to multipath effects, a phenomenon known as multipath lobing occurs, as illustrated in FIG. 2. With increasing elevation angle, for example, the amplitude of the sum signal progresses through peaks and nulls. Likewise, with increasing elevation angle, the difference signal also progresses through peaks and nulls. The peaks of the difference signal correspond to nulls of the sum signal while nulls of the difference signal correspond to peaks of the sum signal. These peaks and nulls correspond to points of maximum constructive and destructive interference, respectively, between direct and multipath echoes.
The multipath distortion of the amplitude and relative phase of the monopulse sum and difference signals cause the multipath error in monopulse elevation angle information. Therefore, the multipath effects of FIG. 2 must be compensated to obtain accurate target elevation angle, altitude and range measurements and the like. Some conventional methods count multipath lobes as a function of target range to solve for target elevation. However, such methods do not completely compensate for multipath effects and require long observation times to measure target elevation angle, altitude and range and the like. Further, some conventional methods use frequency agility to measure the frequency change required to cause a full multipath cycle. However, such methods require wide frequency bandwidths that are not available to low frequency search radars.
FIG. 3 is a diagram for explaining fading cycles caused by multipath effects. Referring to FIG. 3, if a target moves at a constant altitude and speed from left to right in FIG. 2, the amplitude of either the sum or difference signal received by a monopulse radar system has an amplitude determined by fading cycle function F(R), again, a result of periodic constructive and destructive interference between direct and multipath echoes. (One fading cycle is simply the time between consecutive peaks, in the cycle shown in FIG. 3.) The amplitude of the fading cycle function F(R) fluctuates rapidly toward the left of FIG. 3 due to the fact that the target 2 crosses through more multipath lobes in a given time interval or range interval when the target 2 is relatively near to the antenna feed of the monopulse radar system. On the other hand, the amplitude of the fading cycle function F(R) fluctuates less frequently at ranges relatively distant from the monopulse antenna feed since the target 2 is moving more parallel to the multipath lobes of the sum or difference signal.
FIG. 4 is a graph of elevation angle error caused by multipath effects versus track time in a radar system. The elevation angle error generally fluctuates less at higher elevation angles, and fluctuates more at relatively low elevation angles. This is because reflections from the surface of the earth are more acute at lower elevation angles than at higher elevation angles. The elevation error, particularly at low elevation angles, must be compensated or accounted for to obtain accurate target elevation angle, altitude and range measurements and the like.